Medical implants have found applications in many embodiments in modern medical technology. They are used, for example, to support vascular structures, hollow organs and endovascular implants for fastening and temporary fixation of tissue implants and tissue transplants, but also for orthopedic purposes, for example, as nails, plates or screws.
Thus, for example, the implantation of stents has established itself as one of the most effective therapeutic measures in the treatment of vascular disease. One of the most frequent causes of death in the developed world is cardiovascular diseases, whereby coronary diseases have the highest significance. For the treatment of these diseases, intravascular stents are used, for example, balloons or stents are inserted into the affected blood vessel of the patient, and if necessary, implanted in order to expand such and keep it open.
The implant or stent has a basic body made of an implant material. An implant material is typically an inorganic material that is used for a medical application and which interacts with biological systems. A basic requirement for a material used as implant material that comes in contact with the body when used as intended, is its biocompatibility. Biocompatability is understood to mean the ability of a material to provoke an appropriate reaction of the tissue in a specific application. This includes the adaptation of the chemical, physical, biological and morphological surface properties of an implant to the recipient tissue with the goal of a clinically desired interaction. The biocompatibility of the implant material is also dependent on the chronological reaction of the biosystem that receives the implant. Thus, irritations and inflammations occur relatively quickly, which could lead to tissue changes. Biological systems react in various ways depending on the properties of the implant material. According to the reaction of the biosystem, the implant materials can be divided into bioactive, bioinert and degradable/resorbable materials.
However, because of the intravascular intervention, increased thrombus formation can take place, as well as increased proliferation of smooth muscle cells, which can lead to a new stenosis, a restenosis. Overshooting proliferation of scar tissue thereby leads to restenosis in approximately 30-40% of all uncoated stents after a longer period of time.
In order to prevent the risk factors of a restenosis, a number of coatings for stents were developed that are intended to offer increased hemo-compatibility. However, these coated implants have the problem that, as a rule, they have a short shelf-life after having been produced up to the implantation time, i.e. they can only be stored for a short time, or they also require storage conditions such as, for example, storage of the products at 4° C. This leads to increased waste of completed products and thus to an increased economic loss as well as increased costs with respect to energy consumption. Particularly medical implants made of polymeric materials or with coatings of polymeric materials, perhaps loaded with active substances, must be improved with respect to their shelf-life and storage stability.
Further, many of these coated stents that are loaded with active substances have the disadvantage that the active substances are released too slowly at the surrounding implantation site, as well as that their bioavailability of the respective coating materials is too low. Further, in the selection of the dosage of the active substances that are to be released, it is limiting, that the quantity of the active substance that is to be placed on the exterior of the stent is severely limited because the surface area that is available for application at the stent is very small. Stents of biocorrodible magnesium alloys can have the additional problem that the strongly alkalinity that is created as a result of the corrosion of the material, negatively influences the resorption behavior of the active substance that is to be absorbed. Thus, active substances are sometimes used as hydrochlorides when the solubility of the active substance is too low. However, in the strongly alkaline environment that is created, such hydrochlorides are again transformed into difficult to dissolve, deprotonized active substances.
One problem in the use of biocorrodible implants that consist entirely or partially of a metallic material is also that the decomposition products that are created in the corrosion process of the implant and released often have a significant influence on the local pH value and can lead to undesired tissue reactions. Moreover, because of their increased rate of corrosion, these implants often have an implant integrity that is too short at the implant site for the desired application. Particularly in the degradation of Mg-containing biocorrodible implant materials, the pH value in the immediate environment can rise. This rise in the pH value can lead to a phenomenon that is summarized by the term alkalosis. The increase in local pH value thereby leads to an imbalance of the load distribution in the smooth muscle cells surrounding the vascular structure, which can have the effect of increasing tonicity in the area of the implant. This increased pressure on the implant can lead to the premature loss of the integrity of the implant. If the implant is, for example, a stent, a restenosis can occur in the course of such a vasoconstriction in the vascular structure around the stent or to an impairment of the vascular lumen.